Adsorption of Sulfur Dioxide in Cu(II)-Carboxylate Framework Materials: The Role of Ligand Functionalization and Open Metal Sites

The development of efficient sorbent materials for sulfur dioxide (SO2) is of key industrial interest. However, due to the corrosive nature of SO2, conventional porous materials often exhibit poor reversibility and limited uptake toward SO2 sorption. Here, we report high adsorption of SO2 in a series of Cu(II)-carboxylate-based metal–organic framework materials. We describe the impact of ligand functionalization and open metal sites on the uptake and reversibility of SO2 adsorption. Specifically, MFM-101 and MFM-190(F) show fully reversible SO2 adsorption with remarkable capacities of 18.7 and 18.3 mmol g–1, respectively, at 298 K and 1 bar; the former represents the highest reversible uptake of SO2 under ambient conditions among all porous solids reported to date. In situ neutron powder diffraction and synchrotron infrared microspectroscopy enable the direct visualization of binding domains of adsorbed SO2 molecules as well as host–guest binding dynamics. We have found that the combination of open Cu(II) sites and ligand functionalization, together with the size and geometry of metal–ligand cages, plays an integral role in the enhancement of SO2 binding.


■ INTRODUCTION
Fossil fuels will continue to dominate the energy landscape in the decades to come, leading to significant emissions of SO 2 . 1 Air pollution by SO 2 has detrimental effects on both human health and the environment, 2−5 and SO 2 in the atmosphere is thus a major source of pollution and is associated with global climate change. 3,4 SO 2 is also an important industrial feedstock primarily for the manufacture of sulfuric acid, which uses 98% of the total production of SO 2 . 6 Although the state-of-the-art flue-gas desulfurization (FGD) technologies can remove up to 95% SO 2 , they generate a tremendous amount of solid waste, and residual SO 2 can later poison CO 2 scrubbers downstream of FGD processes. 7 Regenerable methods can mitigate the production of waste by recycling the sorbent post SO 2 adsorption, and the recovered SO 2 can be used further for the synthesis of sulfuric acid. Sorbent materials with high SO 2 capacity can be used as a safe host for the transport of SO 2 , eliminating the energy cost for its reduction to elemental sulfur followed by re-oxidation to SO 2 . 6 Traditional porous materials including metal oxides, 8 activated carbons, 9 and zeolites 10 have been tested for SO 2 adsorption. However, these materials tend to demonstrate low SO 2 capacities under ambient conditions (usually in the range of 1−5 mmol g −1 ) owing to their limited surface areas and often they undergo irreversible structural degradation upon the harsh conditions required to remove adsorbed or bound SO 2 . 11 Metal−organic frameworks (MOFs) are promising sorbent materials owing to their exceptional surface area and tuneable pore environments. 12,13 Functionalization of the organic linker and/or incorporation of coordinatively unsaturated metal sites can deliver targeted properties to the resultant MOFs, such as preferential adsorption of H 2 , CH 4 , CO 2 , and light hydrocarbons. 14−18 The use of MOF materials as SO 2 sorbents is currently a rapidly developing field of study. 19−32 MOF-177 exhibits a record high SO 2 uptake of 25.7 mmol g −1 at 298 K and 1 bar, but it shows irreversible structural degradation upon desorption. 22 Considering that over 100,000 MOFs are reported to date, 33 samples that are stable to repeated exposure to SO 2 are still relatively rare, including MFM-300(In) (8.28 mmol g −1 ), 23 MFM-300(Sc) (9.4 mmol g −1 ), 24 DMOF (9.97 mmol g −1 ), 25 NU-1000 (10.9 mmol g −1 ), 26 SIFSIX-1-Cu (11.0 mmol g −1 ), 27 NU-200 (11.7 mmol g −1 ), 28 MFM-601 (12.3 mmol g −1 ), 29 MFM-300(Sc)@EtOH (13.2 mmol g −1 ), 24 MOF-808 (15.3 mmol g −1 ), 30 MFM-170 (17.5 mmol g −1 ), 31 and MIL-101(Cr)-4F(1%) (18.4 mmol g −1 ) 32 (uptake given at 298 K and 1 bar of SO 2 ). Systems incorporating open metal sites that are capable of capturing SO 2 are extremely rare. 31 Thus, the optimization of pore environment in terms of ligand functionalization, implementation of open metal sites, and control of pore geometry is an important approach to achieve reversible, high adsorption of SO 2 .
Here, we report a comprehensive investigation of adsorption of SO 2 in a series of Cu (II)- 4 ] paddlewheels bound to carboxylate donors of the linker and water molecules at the equatorial and axial positions, respectively. Open Cu(II) sites can be generated by removal of the terminally bound water molecules by heating under vacuum. MFM-101 and MFM-190(F), the latter with a fluoro-functionalization, show fully reversible adsorption of SO 2 of 18.7 and 18.3 mmol g −1 at 298 K and 1 bar, respectively; the former represents the highest reversible uptake of SO 2 in porous solids. These two materials also show high stability toward cyclic adsorption and desorption of SO 2 , retaining full crystallinity and uptake capacity over multiple cycles. The other systems show a decrease in uptake, porosity, or crystallinity upon repeated cycles of adsorption− desorption. The host−guest binding interaction and locations of adsorbed SO 2 molecules in MFM-190(F) and MFM-  The gravimetric adsorption isotherms of SO 2 have been recorded for all eight MOFs at 273 and 298 K and from 0 to 1 bar (Figures S11−S18). MFM-101 shows a SO 2 uptake of 20.8 mmol g −1 (or 1.33 g g −1 ) at 273 K and 1.0 bar, exceeding those reported for all leading sorbent materials for SO 2 under the same conditions, such as MFM-170 (19.4 mmol g −1 ), 31 MFM-601 (16.9 mmol g −1 ), 29 and MFM-202a (12.8 mmol g −1 ). 37   MFM-100, MFM-101, and MFM-102 exhibit excess SO 2 adsorption capacities of 7.6, 18.7, and 12.1 mmol g −1 at 298 K and 1 bar (Figure 3a). A 3.7% loss in uptake capacity over 10 cycles of SO 2 adsorption at 298 K is observed in MFM-100 ( Figure S11b) with the post-SO 2 exposure sample showing broadening and loss of Bragg peaks by PXRD due to reduction in crystallinity and structural order of the sample ( Figure  S19a). However, MFM-101 shows fully reversible SO 2 uptake over 10 cycles without any loss in total uptake capacity ( Figure  3c). Indeed, comparison of the PXRD pattern for the assynthesized and post-SO 2 cycling sample of MFM-101 confirmed little change in crystallinity ( Figure S19b). Interestingly, although MFM-102 only shows a 5.6% loss in uptake capacity over 10 cycles of SO 2 adsorption at 298 K  Table S3).
( Figure S13b), comparison of the PXRD patterns of the fresh and post-SO 2 exposure MFM-102 samples shows that there is a significant loss in long-range structural order in the latter ( Figure S19c).
MFM-126 displays an excess SO 2 adsorption capacity of 7.3 mmol g −1 at 298 K and 1 bar (Figure 3a) and shows stable performance over 10 cycles of adsorption−desorption of SO 2 with full reversibility ( Figure S14b). This is also evidenced by the PXRD pattern of the post-SO 2 sample of MFM-126, which fully retains its crystallinity ( Figure S19d). However, compared with other MOFs, MFM-126 shows a much lower uptake capacity of SO 2 , reflecting potentially the lack of open metal sites.
The ability to capture SO 2 at low concentrations by MFM-190(F) and MFM-101 was tested by dynamic breakthrough experiments with SO 2 -containing gas mixtures at 298 K and 1 bar. Both MFM-190(F) and MFM-101 display excellent dynamic adsorption of SO 2 at a low concentration (2500 ppm SO 2 ) ( Figures S20 and S21). In addition, MFM-190(F) and MFM-101 successfully capture SO 2 from a mixture of 15% CO 2 and 2500 ppm SO 2 with the breakthrough time for SO 2 of 190 and 230 min g −1 , respectively (Figure 3g,h). The dynamic selectivities of SO 2 /CO 2 are estimated to be 5.2 and 2.5 for MFM-190(F) and MFM-101, respectively (Table S3). This result further confirms the ability of MFM-190(F) and MFM-101 to selectively capture SO 2 with an efficiency down to <0.1 ppm from 2500 ppm in a single adsorption cycle under dry conditions. Furthermore, the excellent stability of MFM-190(F) has been demonstrated by three cycles of breakthrough separations of SO 2 /N 2 (2500 ppm SO 2 ) ( Figure S20).
The performance of the state-of-the-art porous materials for SO 2 adsorption under ambient conditions is summarized in Table S3 and Figure 3i. Of the MOFs with nbo topology, MFM-101 and MFM-190(F) exhibit the most promising stability and uptake capacity, comparable to those of MIL-101(Cr)-4F(1%), which is the previous record holder for SO 2 adsorption at 298 K and 1 bar for a porous material that is stable to repeated SO 2 cycling. 32 4 ] paddlewheels that define the boundary of the cylindrical and spherical cages, sandwiched between three phenyl rings that link the paddlewheels together. The SO 2 molecule is located closer to one of the [Cu 2 (OOCR) 4 ] paddlewheels than the other two, with a potential side-on interaction between the delocalized π systems of the two neighboring phenyl rings and S SOd 2 [δ+, S SOd 2 ···π left = 3.95(2) Å, S SOd 2 ···π right = 4.42(1) Å]. Site II is further stabilized via twofold hydrogen bonding between O SOd 2 and the hydrogen atom on the phenyl ring [O SOd 2 ···H−R = 2.39(2) and 2.77(2) Å, <Ȯ···H−C = 148 and 134°]. In addition, the dipole−dipole interactions [O SOd 2 ···S SOd 2 = 2.76(1), 3.65(7), and 4.16(1) Å] between SO 2 molecules at Sites I and II further stabilize the packing of SO 2 . Interestingly, the primary and secondary sites for SO 2 are opposite to those found in SO 2 -loaded MFM-170, 31 where the open Cu(II) sites serve as secondary binding sites. One possible explanation for this is that half of the axial positions in MFM-170 are blocked by a pyridyl N-center from the linker, and some steric hindrance may be present around the open Cu(II) site. By contrast, all Cu(II) sites in MFM-190(F) can bind SO 2 , coupled with the presence of additional −F sites, contributing to the enhanced uptake of SO 2 .
Site III (SO 2 /Cu = 0.287) is found at the center of the cylindrical cage, located near the six terminal phenyl rings that connect the central three [Cu 2 (OOCR) 4 ] paddlewheels. The SO 2 molecule is offset to the phenyl ring and stabilized by dipole−dipole interactions between the delocalized π system and S SOd 2 [O SOd 2 ···π system = 3.19(5) Å] and electrostatic interactions between H from the phenyl rings and the O-center from the SO 2 [R−H···O SOd 2 = 2.27(6) and 2.86(1) Å]. Site IV (SO 2 /Cu = 0.232) lies in the window between the cylindrical and spherical cages, sandwiched between two Lewis basic pyridyl rings. Interestingly, similar to that observed in SIFSIX, 28 this binding site is stabilized by a side-on S δ+ ···F δ− electrostatic interaction [S···F = 2.02(1) Å], coupled with twofold hydrogen bonds between the O SOd 2 and the hydrogen atom on the phenyl ring [O SOd 2 ···H−R = 2.11(4) and 3.28(1) Å, <O···H−C = 143 and 139°]. This result confirms that beyond acting as an active site for SO 2 binding, the primary role of the −F group is to increase the framework stability toward SO 2 adsorption, consistent with the notable difference in the adsorption stability between MFM-190(F) and .
A very small amount of SO 2 was loaded into desolvated MFM-126 in order to probe the primary binding site in the absence of an open Cu(II) site. As a result, only one binding site (I′) for SO 2 with a SO 2 /Cu ratio of 0.801 is observed and is located in the window between the larger cylindrical cage and the smaller spherical cage, similar to Site IV in MFM-190(F) ( Figure 5). Site I′ in MFM-126 is stabilized via dipole−dipole interactions between S SOd 2 and the oxygen of the amide group [S SOd 2 ···O�C = 3.77(4) Å] as well as between O SOd 2 and the nitrogen atom of the pyridine ring [O SOd 2 ···N−R = 2.82(2) Å]. Both of these interactions appear to be weaker than those in Site IV of MFM-190(F), consistent with its low adsorption uptake.
Overall, the end-on binding interaction between SO 2 and the open Cu(II) sites located at the axial positions of the [Cu 2 (OOCR) 4 ] paddlewheels play a significant role in the high SO 2 capacity of MFM-190(F), displaying the highest SO 2 /Cu ratio and the strongest host−guest interaction. In contrast, the primary binding site in MFM-126 is only weakly stabilized. Thus, the in situ NPD study has directly rationalized the adsorption performance of these materials.
In  (Figure 6a). Peak I is assigned to the distortion of the phenyl ring and Peaks II and III to the in-plane and out-of-plane bending modes of the aromatic C−H groups, respectively. 38 Peak IV is assigned to the C−F stretching. 39 On dosing with SO 2 (0−1 bar), a blue shift of 6 cm −1 is observed for Peak I to 1562 cm −1 , indicating a stiffening effect of the π system upon binding of SO 2 molecules to the phenyl ring ( Figure 6a). The red shift (Δ = 4 cm −1 ) of Peak II reflects the presence of −CH δ+ ··· δ− OSO supramolecular contacts (Figure 6b). Furthermore, blue shifts of Peaks III and IV to 926 and 900 cm −1 (Δ = 7 and 4 cm −1 , respectively) are observed, consistent with the presence of hydrogen bonds and dipole−dipole interaction as observed in crystallographic studies (Figure 6c).
Two characteristic peaks at 1639 and 1602 cm −1 are observed for desolvated MFM-126 (Figure 6d), which are assigned to the stretching modes of the C�O group and pyrimidine ring, respectively. 38 Red shifts to 1632 and 1594 cm −1 (Δ = 7 and 8 cm −1 , respectively) upon binding of SO 2 suggest dipole−dipole interactions with C�O and C�N bonds, again consistent with the crystallographic model.
Desolvated   (Figures S28−S32) shows a number of characteristic peaks at 1712, 1218, 1027, 838, and 680 cm −1 (denoted as I, II, III, IV, and V). Peak I is assigned to the stretching of the C�O bond and Peaks II and III to the symmetric and asymmetric stretching modes of the C−O bond, respectively. Peaks IV and V are assigned to the in-plane and out-of-plane bending modes of the C−H group of the aromatic ring, respectively. Notable shifts are observed upon SO 2 dosing for Peak I (from 1712 to 1697 cm −1 ), Peak II (from 1218 to 1228 cm −1 ), and Peak III (from 1027 to 1022 cm −1 ), indicating interactions between adsorbed SO 2 molecules and the carboxylate group. 38,40 The blue shifts at Peak IV (from 838 to 850 cm −1 ) and Peak IV (from 680 to 686 cm −1 ) suggest the presence of hydrogen bonds between the aromatic C−H groups and adsorbed SO 2 molecules.
Upon adsorption of SO 2 in MFM-101, four bands experience an obvious shift (Figures S33−S35). The red shift at Peak I from 1637 to 1631 cm −1 suggests distortion of the phenyl ring due upon binding of SO 2 . 38